Not to start any fanboy wars but I kinda agree.
I see it being (in terms of BO)
1: Star Wars Ep. VII
2. Avengers 2
3. Justice League.

Maybe if JL had Bale (the Batman that people became attatched to since 2005) or had some sort of build up I'd think it'd have more of a chance against the other 2. Also Star Wars and Avengers are already sequels to 2 massive blockbusters.

But still JL will make good money I'll be surprised if it makes under 500 mill

I won't be surprised. Justice League will be the Green Lantern of 2015. WB is totally going out of it's way to make it seem like an Avengers rip off. Sure Thanos is based on Dark Seid, but the general audience isn't going to see that.